SOMERSWORTH — The city’s downtown infrastructure project is nearly $145,000 over budget, city officials said this week, the result of costly add-ons and surprise conditions beneath the roadway.

Additions to the initial $4.85 million project include $49,000 to rebuild stairs outside City Hall, $18,700 for temporary paved sidewalks on Main Street, $40,000 for drainage upgrades around FairPoint Communications ducts and $175,000 for improvements to Station Street.

Those expenses have been partially offset by more than $300,000 in cost reductions from eliminating night work and switching some materials.

The latest estimate for the project, which includes new utilities, sidewalk upgrades and fresh pavement on Market, High and parts of other downtown streets, is $5 million.

Nobody at City Hall is happy about the rising costs, which likely will be paid for by tapping cash reserves. Given the scope of the project and the age of the buried utilities, some city officials are surprised the cost hasn’t ballooned even higher.

“I’d say it’s very unusual on a project this size that we wouldn’t have more than this, quite frankly,” said Carol Murray, interim public works director and a former N.H. Department of Transportation commissioner.

The City Council’s Public Works and Environment Committee on Thursday endorsed roughly $140,000 in new spending for the downtown project.

That includes two “change orders” that are essentially bundles of revisions and changes ranging from a few thousand dollars to $10,000 or more.

The committee also endorsed spending $35,000 for sidewalk upgrades on the west side of the five-way intersection at High, West High and Washington streets, and $14,000 to replace 18 cracked manhole rim covers. Another $12,000 is necessary for police details to direct traffic at Washington and Main streets during peak traffic times. The city may switch to civilian flaggers instead of police to save money, however.

These extras, as well as the two change orders, will require approval by the full City Council to take effect. The council meets next on Aug. 11.

Some of the overruns stem from surprise conditions underneath the roadway, from rocky ledges that must be pounded out before new utility lines are installed to an unexpected number of water connections at duplex homes.

Other costs stem from work city councilors wanted to get done that was not included in the original project specifications. That includes $35,000 for sidewalks at the five-way intersection and $175,000 for upgrades to Station Street.

City councilors have argued all along that they would rather the project be done correctly, rather than have to dig into new pavement in a year or two to replace something they could have repaired now.

“It would be pretty embarrassing for the city as a whole if we do a project of that magnitude, that looks beautiful, and in two years we have a collapsing manhole because of a bad rim cover,” Councilor Dale Sprague said Thursday.

Councilor Martin Pepin said he’s frustrated by the overruns. At the same time, he acknowledges it’s impossible to prepare for every contingency when designing a project this big.

“We try to make sure we hire engineers to look for that stuff and give us all the facts we can possibly get, but unfortunately they can’t see everything and know everything that was put in 100 years ago,” Pepin said. “There are things that pop up that we have to deal with.”

Hoyle, Tanner and Associates is project manager for the construction job.

Shawn Reynolds, who is managing the Somersworth project, told the Public Works and Environment Committee Thursday that he didn’t foresee any serious issues that would push costs sharply higher.

Murray agreed the worst probably already has been found.

“It might go up a little bit, but I am not seeing anything big,” she said.

“So much of street opened, and they’ve put so much drainage in, the big ticket items are probably behind us.”